As a bearing (hub bearing) for a car body which can be exemplified as a representative bearing used in environment in which water penetrates thereinto, in the 1980s, to improve operability in mounting it on the car body, car makers came to adopt a double row angular contact ball bearing or a double row tapered roller bearing in which outer rings of a back-to-back duplex bearing were held together as a unit. These hub bearings are called the first-generation hub bearing. By holding the outer rings together as a unit, an initial axial gap was set to a proper value in assembling the hub bearing. Thus it was unnecessary to adjust a preload in mounting the hub bearing on the car body. Thereafter a double row bearing called the second-generation hub bearing was developed by providing the outer ring of the first-generation hub bearing with a flange portion. Because merely arranging two standard bearings has a limitation in a decrease of the weight and size of the hub bearing, the hub bearing, a shaft (hub ring), and a housing (knuckle) which are peripheral parts of the hub bearing were held together as a unit to decrease the number of parts and the weight of the hub bearing. By changing the method of fixing of the hub bearing to the knuckle from press fit to bolt tightening, it became easy to mount the hub bearing on the car body. Further in the third-generation hub bearing, the shaft (hub ring) and the inner ring of the hub bearing are held together as a unit to decrease an excess metal and improve the line-assembling efficiency. Recently the fourth-generation hub joint in which the hub bearing and a constant velocity joint are held together as a unit has been developed.
To facilitate an operation of mounting the automotive hub bearing on the car body and make the hub bearing lightweight and compact, recently the second and third generation hub bearings are increasingly adopted.
Paying attention to the material of the hub bearing, in the first-generation hub bearing, bearing steel (for example, SUJ2) was used for inner and outer rings. Carbon steel for machine structural use such as S53C which has a favorable forgeability and is inexpensive came to be used for the second and third generation hub bearings having the flange mounted on the outer ring. The rolling fatigue strength of the bearing portion is secured by heat-treating the raceway portion consisting of the carbon steel for machine structural use with a high-frequency wave. But the carbon steel for machine structural use has a low surface strength because it contains a small amount of an alloy component and is thus inferior to the bearing steel in the resistance to peeling that occurs from the surface thereof. Therefore when the same lubricating specification as that for the first-generation hub bearing is applied to the second and third generation hub bearings, the second and third generation hub bearings have durability inferior to that of the first-generation hub bearing when lubricating condition is strict.
The automotive hub bearing is operated not only in fine weather but also in very bad use conditions such as in rainy weather and on bad roads and seashores. Although the penetration of water or a foreign matter into the hub bearing is restrained by sealing member, it is impossible to construct the sealing member capable of entirely sealing the hub bearing. Thus the hub bearing is not immune from the penetration of the water or the foreign matter thereinto. Further the hub bearing is demanded to have a low torque from the standpoint of energy saving. As one of methods of allowing the hub bearing to have a low torque, a light contact of the seal with an outer ring or the like is conceivable. Therefore there is a higher possibility that the water penetrates into the bearing and that the lubricating state inside the bearing becomes bad. This problem is common to the hub bearing of each generation. In the hub bearings of the second, third, and fourth generations using the carbon steel for machine structural use as the material thereof, there is a higher possibility that peeling occurs from the surface thereof when the lubricating state is bad.
Regarding the improvement of the water-resistant grease, the decrease of the rotational torque by adopting the low-viscosity base oil (see patent document 1) and the impartation of conductivity for the removal of static electricity (see patent document 2) are known. But no consideration has been made to maintain the performance of the bearing in preparation for the penetration of water into grease.
The penetration of water into the bearing causes the following problems to occur: When waterdrops penetrate into a region thereof to which a load is applied, an oil film is broken and thus disadvantageous in the lubricating performance and metal contact occurs. Consequently there is a danger that the bearing may wear and peel (smearing and the like) from the surface thereof or peel in an early stage. The early-stage peeling means peeling that generates a white tissue change in the neighborhood of the surface of the bearing and also peeling which develops crack in the neighborhood of the surface thereof in a direction in which rolling elements roll and a direction opposite to the rolling direction thereof. In dependence on the state of the presence of the water inside the bearing, rust is generated inside the bearing.
Patent document 1: Japanese Patent Application Laid-Open No. 2003-239999
Patent document 2: Japanese Patent Application Laid-Open No. 2004-169862